GaAs-based as well as InP-based quantum well lasers are known. Such lasers typically comprise an active region that contains one or more quantum wells, the active region sandwiched between two waveguide layers, the combination in turn sandwiched between two cladding layers. Adjacent quantum wells are separated by a barrier layer.
Although quantum well lasers are widely used, prior art quantum well lasers (especially InP-based quantum well lasers) have shortcomings. For instance, in order to overcome problems (e.g, a small conduction band discontinuity between InP and the active region material, as well as a low potential barrier for electrons between the active and cladding regions) inherent in the commonly used InGaAsP/InP material system, it has been found necessary to use relatively highly doped cladding layers. This results in relatively high absorption of the lasing mode, necessitating use of a multi quantum well (MQW) active region. Furthermore, due to the noted and other characteristic of InGaAsP/InP quantum well lasers, such lasers typically exhibit undesirably strong temperature dependence of threshold current and external quantum efficiency. The referred-to strong temperature dependences are particularly detrimental for uncooled lasers which typically have to be operable over a rather large temperature range, e.g., -40 to +85.degree. C. for some uncooled optical communication lasers.
C. E. Zah et al., OFC 94 Technical Digest, Vol. 4, p. 204 disclosed an InP-based strained MQW laser with improved temperature performance. The improvement resulted from the substitution of AllnGaAs for the conventionally used InGaAsP, with attendant increased electron confinement energy. The characteristic temperature (T.sub.o) of the threshold current of the laser was reported to be 80K.
S. Hausser et al., Applied Physics Letters, Vol. 62(7), p. 663 disclosed a InGaAsP/InP MQW laser that comprised an InP hole barrier layer between the MQW region and the n-side cladding layer. The barrier served to prevent hole leakage into the n-side cladding region, and was said to result in improved temperature performance of the laser. To was reported to be 62K. The authors report also fabrication of a laser ". . . that has an InP electron barrier but no hole barrier adjacent to the quantum wells . . .", and concluded that the (InP) hole barrier is very important but that the (InP) electron barrier ". . . has only a minor effect and is not really necessary . . . ".
In view of the potential importance of reduced temperature dependence of laser characteristics, especially for uncooled lasers, it would be highly desirable to have available a laser that can have relatively low temperature dependence of, e.g., threshold current and external quantum efficiency. This application discloses such a laser.